As a result of the ever expanding demand for energy, the world's easily accessible oil reserves become swiftly depleted. The oil and gas industry today has a typical recovery of hydrocarbons with value of 30-40%,1,2 which indicates that the majority of the existing oil remains trapped in the pores of the oil bearing porous media. An increase in recovery efficiency (up to 60-80%)1 will therefore be a key factor for meeting the increasing energy demands. To this end, there is a need for new and more sophisticated mapping and production techniques.
Injection of water, also referred to as water flooding,3 is commonly applied to produce so-called secondary oil, resulting in an increase of the total recovery efficiency up to 50%.4 The recovery results of a water flooding process is largely influenced by the rock and fluid characteristics within the particular reservoir.5 Due to viscosity and capillary effects, water may however bypass confined oil that remains in the reservoir, leading to a so-called water breakthrough,6 in which preferred water pathways are developed in the reservoir connecting injection sites directly to the producer well where the recovery of water overtakes that of the secondary oil; water breakthrough can set in long before depletion of a reservoir.5 
These complex and challenging reservoir conditions require an improved knowledge of the subsurface physical and chemical properties. Reservoir flow characterization is regularly performed using isotope tracers, which are injected with the water flooding process to obtain the flow dynamics in a reservoir.7 This is further extended with complementary techniques to image additional reservoir parameters, such as analysis on production profiles of reservoir fluids, pressure tests, and time lapse seismic examinations.4 
The limitation of the commonly applied isotope tracers is that they primarily provide information on flow characteristics, and often do not possess any physical and chemical sensor functionality.10 In addition, a significant number of reported tracers consist of either toxic compounds or radioactive nuclides.8,9 This limits their use due to health, safety, environmental, and legislation issues.
As any additional information of physical and chemical properties within the reservoir and its fluids can add a significant contribution to improve the production process, there is a quest for an improved sensor system. Key characteristics for these sensors' functionalities are the temperature, amount and nature of dissolved ions, pressure, pH, and reservoir chemistry. As a consequence of the complex and hostile reservoir environment often encountered, many classical sensor materials (e.g., organic chromophores) have shown to be not suitable.10,11 
Recent publications suggested nanomaterials with extended sensor functionality as one next step in reservoir characterization.12,13 An important class of these nanomaterials are the so-called quantum dots (“ODs”). QDs are semiconductor nanocrystals,14 which are not only brightly fluorescent, with a size-tunable fluorescent emission color, but have also proven to be a versatile platform for further functionalization.15 QDs have been used as fluorescent nanomaterials in areas where stability, endurance, and specialized chemical functionalization are crucial. These areas are typically found in biomedical research where robust nano-sensors are demanded, often extended with dedicated surface functionality.15,16 
Another and relative new type of “nano” particles are the so-called noble metal clusters. Their bright optical behavior, which is size-tunable, is to a certain extent comparable to that of QDs.17 Their inert inorganic nature together with their relatively high chemical stability and solution process ability makes these noble metal clusters an interesting material to combine with the earlier discussed QDs in a mixed sensor for reservoir imaging. Water-dispersed nano-sensors are compatible with the commonly applied technique of water flooding and therefore an ideal starting point, as its infrastructure is readily available throughout the oil and gas industry for enhanced oil recovery.3 Extended information about the specific chemical and physical conditions within the reservoir is beneficial to optimize secondary oil production.
Application of fluorescent nanomaterials as sensors added in a water flooding process for reservoir imaging demands a water-dispersible nanoparticle with a controlled stability. Embodiments of the present invention synthesize brightly luminescent InP/ZnS QDs and silver clusters with different emission colors and various water stabilizing surface coatings. The different emission colors are clearly easy to discriminate from each other, which is beneficial for multiplexing in a dedicated sensor composition. The challenging reservoir conditions (e.g., high salinity, high pH, and temperature) have often been found as the limiting factor on the stability of fluorescent materials for reservoir imaging. The use of inorganic chromophors based on the QDs and metal clusters described herein showed an improved stability in these challenging conditions. By applying different surface coatings on the nanomaterials, several sensor applications have been identified with respect to pH, chemical environment, temperature, and presence of solids representative within the reservoir.
In practical situations, the ratio of the photoluminescent intensity of different particles may be measured at the producer well. Direct interpolation of the emission ratios provides a unique fingerprint of the reservoir environment with respect to pH, chemical environment, temperature, and present solids. In addition, as the sensors were developed to function in the water phase, the typical background luminescence of oils showed not to be affected by the sensor functionality.
QDs and silver clusters are described herein as a class of materials for applications as luminescent probes with dedicated sensor functionalities for reservoir imaging, exhibiting stability and a significant freedom for surface chemistry.